The mayor of Moscow is sometimes called the second most powerful official in Russia after President Vladimir Putin. The city of nearly 12 million people is Russia's economic and political capital - think New York City and Washington combined. So the results of yesterday's mayoral election made big news not because, as expected, the race went to the Kremlin-backed incumbent but because the challenger got 27 percent, far better than expected. As NPR's Corey Flintoff reports, the mayor barely won the 51 percent he needed to avoid a runoff.

COREY FLINTOFF, BYLINE: It must have seemed like a good strategy back in June, when incumbent Mayor Sergei Sobyanin called for an early election. He was appointed to his post in 2010, and he could have served two more years in his present term. Sobyanin planned to take advantage of his relative popularity and gain the legitimacy of having won an election, while allowing any potential rivals very little time to organize and campaign against him.

Alexei Zudin of the Institute for Socio-Economic and Political Studies says Sobyanin's advantages seemed so great that he didn't spend much time pressing the flesh with voters.

ALEXEI ZUDIN: (Foreign language spoken)

FLINTOFF: Zudin says Sobyanin's natural constituents, such as conservative pensioners, didn't know he needed the support. The mayor's chief rival was opposition leader Alexei Navalny, who helped organize the mass protests in Moscow in 2011 and 2012. Navalny ran a Western-style campaign that harnessed the energy of more than 15,000 volunteers and raised more than $3 million on the Internet. With relatively low voter turnout, Navalny's get-out-to-vote effort paid off.

NIKOLAI PETROV: And now experts are saying that Sobyanin's usual conformist electorate has been demobilized while Navalny's electorate has been mobilized.

FLINTOFF: That's Nikolai Petrov, a professor of political science at Moscow's Higher School of Economics. Petrov says the incumbent's narrow victory leaves him open to charges of vote fraud, and that's just what Navalny's campaign is calling it. They and the independent vote-monitoring group Golos say exit polls show that Sobyanin got less than 50 percent, which, by law, would force him into a runoff against Navalny.

Analysts say it's unlikely that authorities will agree to that, and they already have a sword hanging over Navalny's head that could keep him out of politics altogether.

This summer, Navalny was convicted of fraud in a trial his supporters say was based on trumped-up charges. He's appealing the five-year prison sentence, but if his conviction is upheld, he could be banned from politics under Russian law.

Petrov says Navalny's strong showing in the election could make it difficult for the Kremlin to take immediate action against him without facing political unrest.

PETROV: Imagine if the person who got almost one-third of votes in Moscow will be put into the jail immediately after getting support of more than 600,000 Muscovites. It will cause huge political protests.

FLINTOFF: Navalny spoke to thousands of supporters tonight at a rally in central Moscow, telling them that he is certain they will get a second round. In his words, a serious political opposition is born in Russia. Corey Flintoff, NPR News, Moscow.

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